In general, the maximum speeds for commercial vehicles (trucks, buses, and the like) are limited by rules. For example, in Korea, the maximum speed of trucks is limited to 90 KPH, the maximum speed of coaches is limited to 110 KPH, and the maximum speed of intra-city buses is limited to 80 KPH.
The legal maximum speed limit of Korean trucks is 90 KPH, and the maximum speed of 90 KPH means that the vehicles of the manufacturers should never exceed the speed limit of 90.0 KPH and are tuned to have a maximum speed in a range of 87.5 KPH to 88.9 KPH.
Some other countries do not limit the maximum speeds of vehicles by such rules, and the maximum speeds of trucks are set to an higher limit (for example, 120 KPH) so as not to reach a tire speed limit (for example, 130 KPH).
Hereinafter, a method of controlling fuel injection for limiting the maximum speed of a lorry according to the related art will be described with reference to FIG. 1.
As illustrated in FIG. 1, if the actual maximum speed of a vehicle exceeds a tuning upper limit and reaches a maximum speed limit, an engine Electronic Control Unit (ECU) performs a fuel cut control such that the vehicle may not be further accelerated.
Furthermore, when the vehicle is accelerated through fuel injection at a tuning lower limit (for example, 87.5 KPH), the amount of injected fuel is controlled such that the vehicle is slowly accelerated to prevent the driver from feeling an acceleration feeling (acceleration shock) by an abrupt acceleration.
Further, even when a fuel cut control is performed at an upper tuning limit (for example, 88.9 KPH) or higher, the speed of the vehicle may instantaneously increase up to 89.5 KPH by an inertial force or a resilient force due to the weight of the vehicle when the vehicle descends, but a fuel map is tuned such that the speed of the vehicle never exceeds 90 KPH.
However, the method of limiting the maximum speed of a vehicle according to the related art has the following problems.
Because vehicles (particularly, trucks) cannot exceed the maximum limit, they may not be accelerated to the maximum limit or higher on a downhill road and the driving speeds of the vehicles cannot be increased even when the vehicles are empty. This is because the drivers cannot arbitrarily set the maximum limits of the vehicles.
The maximum limits of the vehicles are fixed, but the fuel ratio would be improved due to an inertial driving force if the vehicles are accelerated to a degree on a downhill road. Accordingly, the fuel ratio may be improved by using elastic travel of the vehicles when an uphill road is present immediately after a downhill road, and the vehicles can be safely driven by decelerating the vehicles to a degree on a flat road after a downhill road. Driving performance and fuel ratio deteriorate when driving schemes fail to realize these advantages.
Because the actual maximum limit does not need to be restricted in some countries (for example, China) that do not have any maximum limit rules, the maximum limit (for example, 120 KPH) is set according to a tire allowable limit (for example, 130 KPH).
Because the driver can drive at a speed almost close to the maximum limit by setting the maximum limit to a very high value, it may be advantageous in driving efficiency, but fuel ratio deteriorates as driving speed increases.
Furthermore, because the driver drives the vehicle while being careful with the vehicle speed to improve fuel ratio, the driver should frequently check the speedometer.
Although the driving speeds of trucks, which have been produced in some countries (for example, China), have been significantly improved through high engine power, fuel ratio deteriorates due to high driving speed so that the employers (the owners of the vehicles) have complaints, and in contrast, the employed drivers tend to drive at as high speeds as possible because they want to take rests for a longer time.